Without knowing more about your room or preferences, I can only make general recommendations. Plus, I cant make any assurances of the compatibility with your system - that's something you'd have to assess during a demo.
The ProAc D28's are definitely worth listening to. ProAc has been one of our most popular speaker brands for quite a while now, chosen after comparisons to other brands. They seem to have the right blend of warmth and sweetness, and are just the ticket for those wanting to stay away from neutral speakers.
Second up, I would recommend looking at the KEF Reference 203/2's. These are slightly over your budget, but ex demo or B grade would be worth tracking down, as they're so precise in their imaging and vividness that very few other speakers seem to be able to match.
The PMC Twenty.24's are a very open and detailed speaker, with quite a big soundstage, if that sort of thing floats your boat. They do have a lively treble though, so matching needs a little care.
The Monitor Audio GX300's are part of one of the best ranges that I think MA have come up with yet. They've made big steps forward with this range, and it's far better balanced than the Silver series, so you have no worries about them improving greatly over what you already have.
The Naim Ovator S400 (along with the S600), is by far the most accessible speakers that Naim have ever produced. Previously they've always been very dry, but the new Ovators are a lot of speaker for the money, and while one or two areas may just lack a little against the best, their strength is in their BMR midrange, which definitely gives them something over their competitors - it almost sounds like they lack an audible crossover point.
It's worth throwing in the KEF R900's in my opinion as they sound quite different to the Reference models. Their sound is balanced more like the Blades, which is a more accessible sound to many than the Reference.
Just a few recommendations to be getting on with